Dog Fights

[subo]

Currently we have four SPCA addoptees sharing our home. Two we have had for 5 years, and the other two now for 2 years. All have seem to get along well, until this fall. The smallest (& oldest) dog, Lady, got into an unprevoked fight with the middle-aged dog, Summer. Now, Lady is kept inside, seperated from the others (she has the cats and family for lots of attenition). While our others have always been fine, this morning Rose (6) and Winter (3) started fighting. We now have Rose inside (where they sleep at night). I am curious as to some ways to prevent this sort of thing from happening again. The dogs are supervised, have 1 1/2 acres (fenced) to run on (some areas are off-limits), toys, walks, all are fed at a specific time, etc. We don't have any alternative other than keeping the dogs away from each other. Euthanizing a healthy dog is out of the question, as are giving away or surrendering to the SPCA (I used to work in ours, and I don't want to send them there). Any suggestions would be greatly appreciated!

~Tara